Tag: transfer of heat

Quizzes (8) Questions (172)

Questions Related to transfer of heat

Multiple choice physics heat energy transfers heat and heat transfer heat energy transfer transfer of heat

Two thin walled sphere of different materials ,one with double the radius and one fourth wall thickness of the other are filled with ice If the time taken for complete melting of ice in the sphere of larger radius is $25$ minutes and that for smaller one is $16$ minutes,the ratio of thermal conductivities of the materials of larger sphere to the smaller sphere is  

  1. $4:5$

  2. $25:1$

  3. $1:25$

  4. $8:25$
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Using the heat conduction formula Q = (KA dT t) / d, where Q is proportional to mass (volume * density), we relate the time taken to melt ice to the thermal conductivity. The ratio of conductivities is derived from the given radii, thicknesses, and times.

Multiple choice physics heat energy transfers heat and heat transfer heat energy transfer transfer of heat

Iron, lead and brass spheres of equal masses are heated to a common temperature and are kept on a sheet of wax. The sphere that passes through the plate first is (lead 0.03 $\operatorname { cal } / \mathrm { g } / ^ { \circ } \mathrm { C }$, iron 0.11 $\operatorname { cal } / \mathrm { g } / ^ { \circ } \mathrm { C }$, brass 0.08 $\operatorname { cal } / \mathrm { g } / ^ { \circ } \mathrm { C }$) 

  1. lead

  2. brass

  3. iron

  4. all at the same time
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

The rate of heat transfer through the spheres depends on their specific heat capacity. Since they have equal masses and are heated to the same temperature, the one with the lowest specific heat capacity will transfer heat the fastest to the wax, but the question implies they all melt through at the same time if the heat capacity is not the limiting factor or if the setup is idealized.

Multiple choice physics heat energy transfers heat and heat transfer heat energy transfer transfer of heat

Glaciers melt:

  1. first at the bootom due to decress in pressure

  2. first at the top due to iuncrease in pressure

  3. first at the bottom due to increase in pressure

  4. first at the top due to increase in pressure
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

The melting point of ice decreases with an increase in pressure. The weight of the glacier creates higher pressure at the bottom, causing the ice to melt there first.

Multiple choice physics heat energy transfers heat and heat transfer heat energy transfer transfer of heat

Two rods having thermal conductivities in the ratio of 5:3 and having equal length length and equal cross-section are joined by face to face. If the temperature of free end of first rod is $100^oC$ and temperature of  free end of second rod is $20^oc$, then temperature of the junction, is-

  1. $90^oC$

  2. $85^oC$

  3. $70^oC$

  4. $50^oC$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Let the thermal conductivity of first rod be $5 K$

Let the thermal conductivity of second rod be $3 k$
now, sum of heat current flowing through the junction =0
$\begin{array}{l} \frac { { 5KA\left( { 100-T } \right)  } }{ x } +\frac { { 3KA\left( { 20-T } \right)  } }{ x } =0 \ 500-5T+60-3T=0 \ 8T=560 \ \therefore T={ 70^{ 0 } }C \end{array}$

Multiple choice physics heat energy transfers heat and heat transfer heat energy transfer transfer of heat

Two walls of thickness $d _1$ and $d _2$, thermal conductivities $K _1$ and $K _2$ are in contact. In the steady state if the temperatures at the outer surfaces are $T _1$ and $T _2$, the temperature at the common wall will be

  1. $\dfrac{K _1T _1+K _2T _2}{d _1+d _2}$

  2. $\dfrac{K _1T _1d _2+K _2T _2d _1}{K _1d _2+K _2d _1}$

  3. $\dfrac{(K _1d _1+K _2d _2)T _1T _2}{T _1+T _2}$

  4. $\dfrac{K _1d _1T _1+K _2d _2T _2}{K _1d _1+K _2d _2}$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

In steady state, the heat flow rate through both layers must be equal. Solving the equation (K1/d1)(T1 - T) = (K2/d2)(T - T2) for T gives the interface temperature.

Multiple choice physics heat energy transfers heat and heat transfer heat energy transfer transfer of heat

$\triangle { H }^{ \circ  }$ for reaction: 
${ F } _{ 2 }+2HCI\rightarrow 2HF+{ CI } _{ 2 }$
is equal to -352.8kJ. If $\triangle { H } _{ f }^{ \circ  }$ for HF is -268.3 kJ ${ mol }^{ -1 }$ then ${ \triangle H } _{ f }^{ \circ  }$ of HCI would be:- 

  1. -22 kJ ${ mol }^{ -1 }$

  2. 88.0 kJ ${ mol }^{ -1 }$

  3. 01.0 kJ${ mol }^{ -1 }$

  4. None
Reveal answer Fill a bubble to check yourself
C Correct answer
Multiple choice physics heat energy transfers heat and heat transfer heat energy transfer transfer of heat

A hetaed smooth metallic body is allowed to cool in air. Which of the following statements about its heat loss is incorrect?

  1. Convection currents in the air aid the process of heat loss.

  2. Conduction is the main mode of heta loss as the body is a good conductor of heat

  3. The rate of heat loss by thermal radiation is increased due to its metallic surface.

  4. The rate of heat loss decreases as the temperature of teh body nears room temperature
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

The statement that the rate of heat loss by radiation is increased due to a metallic surface is incorrect because metallic surfaces (especially smooth ones) have low emissivity, which reduces radiative heat loss. Convection and conduction are standard modes of heat transfer, and heat loss rate naturally decreases as the body approaches ambient temperature.

Multiple choice physics heat energy transfers heat and heat transfer heat energy transfer transfer of heat

Spheres $P$ and $Q$ are uniformly constructed from the same material which is a good conductor of heat and the radius of $Q$is thrice the radius of $P$. The rate of fall of temperature of $P$ is $x$ times that of $Q$ when both are at the same surface temperature. The value of $x$ is:

  1. $1/4$

  2. $1/3$

  3. $3$

  4. $4$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

The rate of cooling (dT/dt) is proportional to the surface area divided by the volume (A/V). For a sphere, A/V = (4*pi*r^2) / ((4/3)*pi*r^3) = 3/r. Since the radius of Q is 3 times that of P, the rate of cooling for P is 3 times the rate for Q (x = 3/1 = 3).